1. Field of the Invention
The present invention relates to plastic pallets used in situations such as transportation and storage of various articles and the like.
2. Description of the Related Art
Conventionally, when transporting and storing articles, wooden pallets have been used, but recently, plastic pallets are becoming more popular because they have the advantages that they are lighter than wood, do not degrade easily, and do not generate splinters which are characteristic of woods. In particular, the integrally molded pallet where both the upper and lower surfaces are injection molded as one, is used in place of wooden pallets in a wide range of fields because of its high production efficiency.
This type of plastic pallet, as shown for example in FIG. 23, has a rectangular upper plate part 200 and a lower plate part 202, and a plurality of columns 204 connecting the upper plate part 200 and the lower plate part 202, and a pair of fork insertion apertures 206 are formed parallel to each other between these columns 204. In the example of FIG. 23, the fork insertion apertures 206 are formed only on two sides, but there are also models where pairs of fork insertion apertures are formed on all four sides.
The upper plate part 200 is called a deck board, and as shown in FIG. 24, has many beams 208 extending horizontally. Similarly, the lower plate part 202 has many beams 212 extending horizontally. Because the beams 208 that form the upper plate part 200 and the beams 212 that form the lower plate part 202 are arranged shifted alternately, many holes are respectively formed between the beams 208 and between the beams 212. For the beams 208 and 212, the cross-section is generally constructed in the form of a ‘U’ or a ‘T’, and these are devised in order to increase the strength of the upper plate part 200 and the lower plate part 202.
The columns 204 are formed with many vertical walls 210 combined length and breadthwise, and connect the beams 208 and 212. To prevent bending of the vertical walls 210, reinforcing ribs 214 are formed at some places along the vertical walls 210 and connected to the beam 208 and 212.
FIG. 25, illustrates a method disclosed in Japanese Examined Patent Application, Second Publication number Sho 49-582, as one example of a molding method for this kind of integrally molded pallet. As shown in this figure, because the beam 208 and 212 pairs are displaced horizontally, even if complicated slide cores are not multiply used, this can be formed relatively easily between an upper mold 216 and a lower mold 218. In addition, compared to the case where the upper and lower parts are formed separately and the pallet then formed by joining the two together, the time-consuming joining process is unnecessary. Therefore, the production efficiency of the integrally molded pallet is high.
However, in the case where the pallet is loaded with opposite ends of the pallet supported, in the above-mentioned plastic pallet, there is a problem in that because the four corners of the fork insertion aperture 206 are subjected to a large bending stress, the fork insertion aperture 206 are easily deformed into a parallelogram, and the entire pallet bends easily compared to the wooden pallet.